The present invention relates to a system for correcting image data in an image scanner of a camera type having a line image sensor for obtaining image data, and more particularly to a correcting system where the image data are properly corrected, thereby preventing distortion of shading.
The line image sensor has a plurality of photoelectric conversion elements aligned on the surface thereof.
In a copy machine or a facsimile machine, a scanner of a flat bed type is used. The scanner comprises a light source and a CCD sensor which are integrally assembled in plane. An image of a draft is lit up with the light source and the light reflected from the draft is applied to the CCD sensor. The CCD sensor is moved in plane along the entire surface of the draft to scan the plane image.
FIG. 12 shows a conventional system having a scanner of a flat bed type. A scanner 1 comprises a light source 2 such as a fluorescent lamp and a CCD sensor 3. An image draft 4 is put on a transparent panel (not shown) in parallel with the scanner 1 which is movable in the direction shown by arrows. The light emitted from the light source 2 illuminates the draft 4 and the reflected light is applied to the CCD sensor 3 to produce a data signal of the image.
In such a system, since the light source 2 emits the rectilinear light, the quantity of light becomes poor at peripheral portions of the draft 4, or the CCD sensor 3 may have unequal sensitivity distribution. Such defects cause distortion of shading. Therefore, it is necessary to correct a white level of the output signal of the CCD sensor to correct the shading.
For correcting the distortion, a standard white sheet 5 is provided. The standard white sheet 5 is read by the CCD sensor 3 every time before the draft 4 is read. Thus, correction data including the unequal distribution of the light quantity and the unequal sensitivity distribution of the CCD sensor 3 is obtained. The actual data of the draft 4 is corrected in accordance with the correction data.
On the other hand, a scanner of a camera type has been developed recently. FIG. 13 shows another conventional system having a camera type scanner. A camera type scanner 9 comprises a line image sensor 8 to be moved in the directions shown by arrows and an optical system such as a lens 6 disposed in front of the line image sensor 8. An object 7 is illuminated with beams of light emitted from light sources 10 and the reflected light is applied to the line image sensor 8 through the lens 6.
The line image sensor 8 has a photoelectric conversion portion corresponding to a predetermined number of pixels per one line. The sensor produces line data in accordance with electric signals of the photoelectric conversion portion. The sensor scans pixels on a predetermined number of lines so that image data can be obtained.
Thus, in the camera type scanner 9, it is possible to read an image not only of a plane object but also a cubic object. The position of the light source 10 is desirably set in dependency on the shape and size of the object.
In particular, in the case that the image of the cubic object is read by the scanner 9, it is not necessary to consider the inequality of illumination caused by the light source and the shade, because such defects represent meanings for the object. However, it is necessary to correct unequal distribution of quality of the lens 6 as well as the unequal distribution of sensitivity of the line image sensor 8.
In order to correct the distortion of shading caused by the lens 6 and the sensor 8, if the standard white sheet 5 is used, problems arise as follows.
The line image sensor 8 has photoelectric conversion section disposed in the first dimensional direction. The sensor is moved in each line by a step motor in the direction perpendicular to the first dimensional direction in synchronism with a reading operation of the image, so that the image data for one sheet picture is obtained. Consequently, the unequal distribution of the lens quality spreads in the second dimensional direction. Accordingly, it is necessary to obtain the correction data of the unequal distribution of the whole area of the one sheet picture.
If the image of the whole area is composed of pixels of 5000.times.4000, and data of 8 bit is allotted for one pixel, correction data of
5000.times.4000.times.8.times.3.fwdarw. about 60 Mbyte
for the three primary colors R, G and B, and a memory having a capacity for storing the correction data must be provided.
Namely, only in the case of the inequality of light of the lens 6, the amount of data becomes extremely large. Therefore, in the system wherein the correction data for the unequal light is complemented before reading the image data, it takes a long time to read data, resulting in decreasing of operability. In addition, in order to provide a memory having such a large capacity, the scale of the circuit is increased, which is on obstacle for miniaturizing the system and reducing the manufacturing cost thereof.
On the other hand, the CCD sensor has an inherent defect that a dark current is produced. If the CCD sensor is driven at a high speed, the temperature of the sensor increases, which causes the dark current to increase. In order to exactly read a half tone of a picture, it is necessary to correct a black level of an output signal of the CCD sensor before reading the data.
There are two methods for correcting the black level. One of the methods is to use a standard black level sheet, and the other method is to shield the outside light.
Japanese Patent Publication 5-36987 discloses a method for correcting a black level using the standard black sheet, in order to reduce an error in the black level correction data caused by dust stuck to the standard sheet.
FIG. 14 shows a conventional scanner system having a standard black level sheet 5'. The image draft 4 is put on a transparent panel 14 such as a glass in parallel with the CCD sensor 3 which is movable in the direction shown by an arrow. The standard black level sheet 5' is provided on the panel 14. The standard black level sheet 5' is read by the CCD sensor 3 every time before the draft 4 is read. Thus, the black level is corrected based on an output of the CCD sensor 3.
FIG. 15 shows the other method. A camera type scanner system 15 comprises a CCD sensor 16 applied with a beam through a lens 11. A total reflection mirror 12 is provided between the lens 11 and the sensor 16. The mirror 12 is moved down for reflecting the light therefrom, thereby shielding the sensor from the light.
FIG. 16 shows another method for shielding the light. A light shielding plate 13 is provided in an end portion of a scanning area of the CCD sensor 16.
In the method, the black level is corrected based on an output of the CCD sensor 16.
However, in the former method, it is necessary to read the standard black level sheet 5' before the draft 4 is read in order to obtain image data. Consequently, it takes a long time to read the draft 4 by the CCD sensor 3. Furthermore, it is necessary to provide a scanning area for reading the standard sheet 5' in addition to the scanning area for reading the draft 4. Since the scanning area is increased, the system can not be made into a small scale.
In the latter method, it is also necessary to obtain a correction data before the image data is taken. Therefore, it takes a long time to obtain image data. Furthermore, it is difficult to completely shield the CCD sensor 16 from the light. Since the light may leak to inside portions of the total reflection mirror 12 or the light shielding plate 13 by diffraction, the CCD sensor 16 is not completely shielded. As a result, the black level is not properly corrected.
In the method of FIG. 15, it is necessary to provide a mechanism to move the mirror 12. Consequently, the system can not be simplified in structure. In the method of FIG. 16, since it is necessary to provide a scanning area for the shielding plate 13 in addition to the scanning area for the image data, the scanning area is increased. Therefore, the system can not be made in a small scale.
Furthermore, in both methods, the level of the output for the correction data in a dark period is extremely low in any methods. Consequently, it is difficult to obtain the output for the dark period at a good S/N ratio.
As a further method, in the camera type scanner system 15 of FIG. 15, the lens 11 is covered with a cap, or the system is disposed in a dark room. However, since such working must be done at every black level correction, the operability of the system is reduced.